Arnold Chiari malformation is a condition in which the contents of the posterior fossa are herniated below the level of the foramen magnum, and it occurs in three basic forms. Patients with this condition frequently have obstructive hydrocephalus which requires a ventriculo-peritoneal shunt. This article describes the case of a infant female patient with Arnold Chiari Type I, who suffered an episode of acute hydrocephalus and neurologic deterioration after ventriculo-peritoneal shunt malformation. A shunt revision did not reverse her neurologic deficits, so a posterior fossa decompression was performed which did improve her condition.